1. Field of the Invention
This invention relates to a developing apparatus for developing an electrostatic latent image formed on an image bearing member, by adhering thereto a non-magnetic one-component toner formed in thin layer on a developing roller to render the electrostatic latent image visible as a toner image; a developing roller for electrophotography used in the developing apparatus; and an apparatus unit and an image-forming apparatus which make use of such a developing roller.
2. Related Background Art
As developers used in electrophotographic developing apparatus for forming black-and-white images, toners endowed with magnetic properties and comprised of a single component (magnetic one-component toners) are conventionally used. Toners having magnetic properties, however, are not suited for color toners. Accordingly, in currently available electrophotographic developing apparatuses for forming color images, toners having no magnetic properties and comprised of a single component (non-magnetic one-component toners) are chiefly used.
Electrophotographic developing apparatus are constructed in a little different ways depending on the types of toners used. In particular, the way in which the toner is carried on the surface of a developing roller (developer-carrying member) differs depending on whether toners are magnetic or non-magnetic. More specifically, in the case when the magnetic one-component toners are used, a magnet is provided within the developing roller so that the toner can be carried, and transported, on the developing roller chiefly by the aid of a magnetic force. On the other hand, in the case when the non-magnetic one-component toners, having no magnetic properties and comprised of a single component, the toner must be carried, and transported, on the surface of the developing roller chiefly by the charging of toner itself, in place of the magnetic force, by the aid of image force which is Coulomb force acting between electric charges on the toner and those generated on the roller surface by the charging. Accordingly, in the case when the non-magnetic one-component toner is used, the magnet is no longer required but instead a means by which the charge quantity necessary for producing the image force is imparted to the toner is required to make the toner carried on the developing roller.
As a commonly available example of a conventional electrophotographic developing apparatus making use of a non-magnetic one-component toner, a contact type developing apparatus is shown in FIG. 4.
As shown in FIG. 4, a developing apparatus 101 has a developing roller 102 which comes into contact with a photosensitive drum (image bearing member) 100 rotated in the direction of X in the drawing to perform development while being rotated in the direction of Y in the drawing, a toner feed roller 104 which is rotated in the direction of Z to feed a non-magnetic one-component toner Txe2x80x2 to the developing roller 102, a developing blade (toner regulation means) 103 which regulates the quantity of the toner Txe2x80x2 to be coated on the developing roller 102 and the charge quantity thereof, and an agitating member 105 which agitates the toner Txe2x80x2 and also feeds it to the toner feed roller 104. In a contact type developing apparatus in which the photosensitive drum 100 is a rigid body and which performs development while bringing this drum and the developing roller 102 into contact with each other in the zone shown by S in FIG. 4, the developing roller 102 may preferably be a roller having an elasticity so that the photosensitive drum 100 and the developing roller 102 can be in close contact without any gap between them. In a developing apparatus having a developing roller 102 formed of a resin which is an elastic material, a developing blade 103 made of a metal, having a good performance of charging by friction, may preferably be used in order to control the quantity of charge to the non-magnetic one-component toner Txe2x80x2.
In the developing apparatus 101, a DC component development bias is applied to the developing roller 102 from a power source (not shown) to form a development potential at the developing zone lying between the photosensitive drum 100 and the developing roller 102, whereby the toner Txe2x80x2 is caused to adhere to the surface of the photosensitive drum 100. More specifically, the toner Txe2x80x2 having been charged adheres to the surface of the photosensitive drum 100 by the aid of Coulomb force, in a pattern corresponding to an electrostatic latent image formed on the surface of the photosensitive drum 100 by an exposure means (not shown), and the electrostatic latent image is rendered visible as a toner image to effect development. The toner having not participated in the development and remained on the surface of the developing roller 102 is taken off by the toner feed roller 104 and is collected in the developing apparatus 101.
This developing apparatus 101 uses an insulating non-magnetic one-component toner basically. For this toner Txe2x80x2 to be carried and transported on the developing roller 102, it is necessary to charge the toner Txe2x80x2 to produce the image force between the toner Txe2x80x2 and the developing roller 102.
Now, a method of carrying the toner Txe2x80x2 on the developing roller 102 will be more detailed below. The toner feed roller 104 feeds the toner Txe2x80x2 to the developing roller 102 and also triboelectrically charge the toner Txe2x80x2 at the contact nip zone between this developing roller 102 and the toner feed roller 104. More specifically, as the toner feed roller 104 is rotated, the toner Txe2x80x2 is guided to the contact nip zone between the developing roller 102 and the toner feed roller 104, and is charged by its friction with the developing roller 102. As the result, the charge quantity necessary for producing the image force by the aid of which the toner Txe2x80x2 is carried on the developing roller 102 is imparted to the toner Txe2x80x2. In that course, the quantity of the toner Txe2x80x2 to be fed to the developing roller 102 is controlled by appropriately setting a difference in peripheral speed between the developing roller 102 and the toner feed roller 104.
In an image-forming apparatus where the above conventional developing apparatus is used, a spherical toner Txe2x80x2 having a uniformly round particle shape has come to be used in order to make reproduced images have a high quality. More specifically, when the toner Txe2x80x2 has an uneven particle shape as in the case of pulverization toners conventionally used, particles having different shapes move in different ways at the time of development, and hence part of the toner Txe2x80x2 may scatter and may adhere to non-image areas (to cause a phenomenon of what is called fog). Use of spherical toner Txe2x80x2 can make such an inconvenience less occur.
The pulverization toners have so high a frictional force of toner itself that, even when the developing roller 102 is constituted of a silicone rubber single layer as shown in FIG. 5, the intended charge quantity can be obtained by its friction with such a silicone rubber surface layer. When, however, the toner Txe2x80x2 is made to have a spherical particle shape, the toner Txe2x80x2 itself has a low frictional force, so that the charge quantity to be obtained by the friction between the toner Txe2x80x2 and the silicone rubber surface layer may lower to make it difficult to obtain the intended charge quantity.
A toner Txe2x80x2 having a core/shell structure encapsulating a wax having a low melting temperature also has come to be used in order to reduce heat energy required in the step of permanently fixing toner images transferred to a recording medium surface (to achieve what is called energy-saved fixing).
A spherical toner Txe2x80x2 having such wax-encapsulated core/shell structure tends to deteriorate because of stress. Accordingly, it has become necessary to lower the hardness of the developing roller 102 and further to lower the coefficient of dynamic friction of the developing roller 102 surface so that the spherical toner Txe2x80x2 deteriorates less. This has made it more difficult to obtain the desired toner charge quantity.
The coefficient of dynamic friction of the developing roller 102 surface must be made low for the following reason. Where the developing roller 102 has a low hardness, in particular, an Asker-C hardness of about 40 degrees or lower as measured with Asker-C Hardness Meter (trade name; manufactured by Kohbunshi Keiki K.K.), the developing roller 102 may vibrate at the contact zone between the developing roller 102 and the photosensitive drum 100 if the developing roller 102 has a high coefficient of dynamic friction at the surface, so that the toner Txe2x80x2 carried thereon may scatter and this effect may appear on reproduced images, resulting in a very low image quality. In order to prevent this, the surface of the developing roller 102 must be made to have a low coefficient of dynamic friction. Here, this problem can be eliminated if the developing roller 102 is made to have a high hardness, e.g., a high hardness of about 45 degrees as hardness prescribed in JIS A, but resulting in a great deterioration of the spherical toner Txe2x80x2.
Accordingly, in place of the developing roller 102, it has become necessary to use, as shown in FIG. 6, a developing roller 112 having a charge-providing layer 112d having a low coefficient of dynamic friction and also having a high charge-providing performance to the spherical toner Txe2x80x2, formed on the roller surface. According to studies made by the present applicants, as materials for this charge-providing layer 112d, resin materials capable of charging the toner Txe2x80x2 negatively by triboelectric charging with the toner Txe2x80x2 and being positively chargeable in and of themselves, as typified by acrylic urethane resins, acrylic polyester urethane resins and polyamide resins. As materials for a base layer 112b of the developing roller 112, silicone rubber is used, as having good rubber properties, e.g., a high durability and a low compression set.
In order to form an electric field across the photosensitive drum 100 surface and the developing roller 112 surface, the developing roller 112 must be made conductive across a mandrel 112a and the surface layer of the developing roller 112. Accordingly, as a developing roller 112 in which conductive particles such as metal oxide particles or carbon particles are dispersed in an appropriate quantity in the chief component of roller constituent members to have a conductivity, a roller having a net resistivity (resistivity across the mandrel 112a and the developing roller 112 surface) of commonly about 104xcexa9 to 109xcexa9 is used.
Here, if the charge-providing layer 112d has a high resistivity, the toner Txe2x80x2 having adhered to the charge-providing layer 112d has a large image force acting on electric charges generated by charging, and the toner Txe2x80x2 may adhere to the charge-providing layer 112d surface strongly, so that the toner Txe2x80x2 may come away from the developing roller 112 surface with difficulty. For example, if the charge-providing layer 112d has a volume resistivity of about 1010 xcexa9xc2x7cm or above, the toner Txe2x80x2 having not participated in the development is not taken off even though it has reached the toner feed roller 104, so that the toner Txe2x80x2 having remained on the developing roller 112 surface many times passes the contact nip zone between the toner feed roller 104 and the developing roller 112 and the contact nip zone between the developing blade 103 and the developing roller 112. As a result of friction at these contact nip zones, the toner Txe2x80x2 is further charged up to become difficult for itself to participate in development, resulting in a decrease in density of reproduced images. Also, even if the toner Txe2x80x2 was taken off from the developing roller 112 surface, electric charges are accumulated in the high-resistance charge-providing layer 112d to block the feeding of any fresh toner Txe2x80x2. As the result, the toner Txe2x80x2 adhering to the developing roller 112 surface may decrease to cause a decrease in density of reproduced images.
If the charge-providing layer 112d is made to have a resistivity of about 109 xcexa9xc2x7cm or above in normal environment, any changes in temperature and humidity tends to cause changes in the conductivity because of a low density of the conductive particles which are dispersed in the chief component in order to provide the conductivity. Hence, the resistivity of the charge-providing layer 112d tends to be affected by the temperature and humidity, so that the resistivity may vary about ten to about hundred times because of environmental changes. For example, there is a possibility that those having a resistivity of about 109 xcexa9xc2x7cm in normal environment come to have a resistivity of about 1010 xcexa9xc2x7cm in an environment of low humidity, and about 108 xcexa9xc2x7cm in an environment of high humidity.
Hence, the upper-limit value of volume resistivity of the charge-providing layer 112d is about 108 xcexa9xc2x7cm.
As for the lower-limit value of volume resistivity of the charge-providing layer 112d, it is determined as a value at which the developing roller 102 can be prevented from being adversely affected by the flowing of electricity to the photosensitive drum 100 surface, and there is no problem as long as it is a volume resistivity of about 104 xcexa9xc2x7cm or above.
More specifically, as the resistivity of the charge-providing layer 112d, a resistivity of approximately from 104 xcexa9xc2x7cm to 108 xcexa9xc2x7cm in volume resistivity is suitable.
In order to form the charge-providing layer 112d on the surface layer of the base layer 112b, an adhesive 112c is required for bonding the both layers because the silicone rubber layer has a low surface energy. In general, as this adhesive 112c, an amino type silane coupling material is used, and it is coated in a thickness of about 1 xcexcm or smaller.
The developing roller 112 must be endowed with conductivity across the mandrel 112a and the surface layer of the developing roller 112 as mentioned previously. The adhesive 112c, however, is in so small a coating weight that it little affects the net resistivity of the developing roller 112 even without being made conductive. More specifically, the conductivity can be imparted across the mandrel 112a and the developing roller 112 surface as long as the conductivity is imparted to the charge-providing layer 112d and the base layer 112b. For example, where the charge-providing layer 112d and the base layer 112b are made to have a volume resistivity of from 105 xcexa9xc2x7cm to 106 xcexa9xc2x7cm, a developing roller 112 having a charge-providing layer 112d of few xcexcm to about 50 xcexcm thick and a base layer 112b of from about 1 mm to about 5 mm thick have a net resistivity of from about 104xcexa9 to about 105xcexa9 when the potential difference between the mandrel 112a and the developing roller 112 surface is about 300 V.
However, in the use of such a developing roller 112 having a multi-layer construction, there is the following problem.
As described previously, the developing apparatus shown in FIG. 4 employs a contact developing system, in which the toner Txe2x80x2 is caused to adhere to the photosensitive drum 100 while keeping the developing roller 112 in contact with the photosensitive drum 100. Also, in order to obtain a sufficient image density, a difference in peripheral speed is commonly provided between the peripheral speed of the photosensitive drum 100 and the peripheral speed of the developing roller 112. Hence, a frictional force acts between the developing roller 112 and the photosensitive drum 100 at their contact nip zone, and a stress is applied to the developing roller 112. Meanwhile, in the developing roller 112 shown in FIG. 6, the base layer 112b is a silicone rubber layer with a low hardness and also the charge-providing layer 112d is a resin layer which is harder than the silicone rubber layer. Hence, the amount of deformation due to stress differs between the charge-providing layer 112d and the base layer 112b, so that a force is applied in the direction where the charge-providing layer 112d is peeled from the base layer 112b. Moreover, since silicone rubber has properties that it has a low surface energy, there is a problem that the charge-providing layer 112d may locally separate from or peel off (i.e., lift) the surface of the base layer 112b because of the stress given to the developing roller 112.
If, taking account of the use of the spherical toner Txe2x80x2 as described previously, which is weak to stress, the base layer 112b is made to have a low hardness (e.g., about 40 degrees or lower as measured with Asker-C Hardness Meter) in order to reduce the stress to the toner Txe2x80x2, a great difference in the amount of deformation may result between the charge-providing layer 112d and the base layer 112b, so that the former may more tend to separate locally from the latter.
Here, one may contemplate to prevent such local separation or peeling by imparting rubber properties to the charge-providing layer 112d so as to absorb the stress. According to studies made by the present applicants, however, the charge-providing layer 112d not only is required to have properties that it has a high charge-providing performance to the spherical toner Txe2x80x2, but also must have a low coefficient of surface friction. Hence, it is not suitable to impart rubber properties to the charge-providing layer 112d, which provide a high coefficient of surface friction.
Where the charge-providing layer 112d has locally separated from the base layer 112b, the following problems occur.
A first problem is that the charge-providing layer 112d may come off from the surface of the developing roller 112. Once the charge-providing layer 112d has come off, for example the charge-providing performance of the developing roller 112 is lost and the photosensitive drum 100 and the developing roller 112 come into faulty contact, to cause extremely faulty images.
A second problem is that the developing roller 112 comes to have a high electrical resistivity. More specifically, where the charge-providing layer 112d has locally separated from the base layer 112b, a gap is formed between the charge-providing layer 112d and the base layer 112b at the separated region, and this gap stands resistant to cause an increase in electrical resistivity, resulting in a high net resistivity of the developing roller 112 at that part. Hence, the developing electric field decreases at the separated region, and also the resistance increases at the surface portion of the developing roller 112, and hence it becomes difficult to take off the toner Txe2x80x2 from the developing roller 112 surface, resulting in a decrease in density of output images at the part corresponding to the separated region. Moreover, like the case where the charge-providing layer 112d has a high resistance, the toner Txe2x80x2 may adhere to the developing roller 112 in a small quantity at its surface corresponding to the separated region, resulting in a greater decrease in image density.
The above problems caused by the local separation of the charge-providing layer 112d from the base layer 112b may become more conspicuous when a developing roller 112 having a low hardness is used. The reason therefor is presumed in the following way.
In the case when the base layer 112b has a high hardness, e.g., about 40 degrees as measured with a JIS-A hardness meter, the contact between the developing roller 112 and the photosensitive drum 100 at the contact nip zone is at a relatively high pressure. Hence, where the photosensitive drum 100 stands in contact with the developing roller 112, contact points at which the charge-providing layer 112d comes into contact with the base layer 112b are formed at the gap portion caused by the local separation between them. At such contact points, the conductivity is restored to make the net resistivity low, and hence any effect caused by the local separation can be smaller than in the case of the low-hardness developing roller. However, the developing roller 112 having this hardness can not be used in the developing apparatus making use of the spherical toner Txe2x80x2 which tends to deteriorate.
On the other hand, where the base layer 112b has a low hardness, the contact between the developing roller 112 and the photosensitive drum 100 at the contact nip zone is at a low pressure. Hence, the contact points between the charge-providing layer 112d and the base layer 112b as stated above are in a small number, and hence the effect caused by the local separation is great.
Thus, making the base layer 112b have a low hardness tends to cause the local separation of the charge-providing layer 112d from the base layer 112b and also makes the effects of separation great. In other words, it is difficult to make the developing roller 112 with the charge-providing layer 112d have a low hardness.
As discussed in the foregoing, in conventional developing apparatus, it has been difficult to obtain a developing roller 112 which has a base layer 112b having a low-hardness and a charge-providing layer 112d having a high charge-providing performance and also having a low coefficient of dynamic friction. As the result, in the developing apparatus making use of the non-magnetic one-component toner Txe2x80x2 having a low melting point and a spherical particle shape, it has been difficult to obtain a developing roller 112 that can perform development without causing any deterioration of the spherical toner Txe2x80x2 and also while carrying the spherical toner Txe2x80x2 well.
To solve the problems as discussed above, as disclosed in Japanese Patent Application Laid-open No. 10-3210, a developing roller is provided in which an elastic layer of a conductive rubber comprised of a resin composition, having a contact angle to water of 75 to 85 degrees, is formed around a shaft made of a metal and a surface protective layer comprised of a resin composition composed chiefly of a fluorine resin, having a contact angle to water of 90 degrees or smaller, is formed on the former""s layer surface. In this proposal, the adhesion between the conductive rubber elastic layer and the surface protective layer is more improved than the conventional, but the problem of separation of the surface protective layer from the conductive rubber elastic layer has not fundamentally been solved. Thus, in the case when the developing roller is used under such development conditions that the difference in peripheral speed is provided between the developing roller and the photosensitive member as stated previously, and the conductive rubber elastic layer is made to have a low hardness so as to be applicable to the spherical toner, it is sought to make an improvement such that the separation of the surface protective layer from the conductive rubber elastic layer may more hardly occur.
An object of the present invention is to provide a developing roller for electrophotography the surface of which has so low a coefficient of dynamic friction and so low a hardness as to apply only a low stress to the toner Txe2x80x2, also having a high charge-providing performance to the toner, and a developing apparatus, an apparatus unit and an image-forming apparatus which make use of such a developing roller.
To achieve the above object, the present invention provides a developing roller for electrophotography, comprising;
a conductive mandrel;
a charge-providing layer having a charge-providing performance to a non-magnetic one-component toner, formed at the surface of the roller;
a base layer having an elasticity, formed at a position nearer to the mandrel of the roller than the charge-providing layer; and
an elastic intermediate layer formed at a position between the base layer and the charge-providing layer;
a composition for forming the elastic intermediate layer having a contact angle to water which is smaller than the contact angle to water of a composition for forming the base layer.
The present invention also provides a developing apparatus comprising;
a toner hopper for holding therein a non-magnetic one-component toner for developing an electrostatic latent image formed on the surface of an image bearing member; and
a developing roller for transporting the non-magnetic one-component toner held in the toner hopper, while causing the toner to adhere to the roller surface;
wherein;
the developing roller comprises;
a conductive mandrel;
a charge-providing layer having a charge-providing performance to the toner, formed at the surface of the roller;
a base layer having an elasticity, formed at a position nearer to the mandrel of the roller than the charge-providing layer; and
an elastic intermediate layer formed at a position between the base layer and the charge-providing layer; and
a composition for forming the elastic intermediate layer has a contact angle to water which is smaller than the contact angle to water of a composition for forming the base layer.
The present invention still also provides an apparatus unit detachably mountable to the main body of an image-forming apparatus, comprising;
a toner hopper for holding therein a non-magnetic one-component toner for developing an electrostatic latent image formed on the surface of an image bearing member; and
a developing roller for transporting the non-magnetic one-component toner held in the toner hopper, while causing the toner to adhere to the roller surface;
wherein;
the developing roller comprises;
a conductive mandrel;
a charge-providing layer having a charge-providing performance to the toner, formed at the surface of the roller;
a base layer having an elasticity, formed at a position nearer to the mandrel of the roller than the charge-providing layer; and
an elastic intermediate layer formed at a position between the base layer and the charge-providing layer; and
a composition for forming the elastic intermediate layer has a contact angle to water which is smaller than the contact angle to water of a composition for forming the base layer.
The present invention further provides an image-forming apparatus comprising;
(I) a plurality of image-forming units each having;
an image bearing member for holding thereon an electrostatic latent image;
a charging assembly for charging the image bearing member primarily;
an exposure assembly for forming the electrostatic latent image on the image bearing member having primarily been charged; and
a developing apparatus for developing the electrostatic latent image by the use of a non-magnetic toner to form a toner image; and
(II) a transfer assembly for sequentially transferring to a transfer medium the toner images formed in the plurality of the image-forming units;
the developing apparatus comprising;
a toner hopper for holding therein a non-magnetic one-component toner for developing the electrostatic latent image formed on the surface of the image bearing member; and
a developing roller for transporting the non-magnetic one-component toner held in the toner hopper, while causing the toner to adhere to the roller surface;
wherein;
the developing roller comprises;
a conductive mandrel;
a charge-providing layer having a charge-providing performance to the toner, formed at the surface of the roller;
a base layer having an elasticity, formed at a position nearer to the mandrel of the roller than the charge-providing layer; and
an elastic intermediate layer formed at a position between the base layer and the charge-providing layer; and
a composition for forming the elastic intermediate layer has a contact angle to water which is smaller than the contact angle to water of a composition for forming the base layer.
The present invention still further provides an image-forming apparatus comprising;
(I) an image bearing member for holding thereon an electrostatic latent image;
(II) a charging assembly for charging the image bearing member primarily;
(III) an exposure assembly for forming the electrostatic latent image on the image bearing member having primarily been charged;
(IV) a plurality of developing apparatus each for developing the electrostatic latent image by the use of a non-magnetic toner to form a toner image;
(V) an intermediate transfer member for sequentially transferring thereto the toner image formed by each of the developing apparatus; and
(VI) a transfer assembly for transferring to a transfer medium at one time a multiple toner image transferred to the intermediate transfer member;
the developing apparatus comprising;
a toner hopper for holding therein a non-magnetic one-component toner for developing the electrostatic latent image formed on the surface of the image bearing member; and
a developing roller for transporting the non-magnetic one-component toner held in the toner hopper, while causing the toner to adhere to the roller surface;
wherein;
the developing roller comprises;
a conductive mandrel;
a charge-providing layer having a charge-providing performance to the toner, formed at the surface of the roller;
a base layer having an elasticity, formed at a position nearer to the mandrel of the roller than the charge-providing layer; and
an elastic intermediate layer formed at a position between the base layer and the charge-providing layer; and
a composition for forming the elastic intermediate layer has a contact angle to water which is smaller than the contact angle to water of a composition for forming the base layer.
The present invention still further provides an image-forming apparatus comprising;
(I) an image bearing member for holding thereon an electrostatic latent image;
(II) a charging assembly for charging the image bearing member primarily;
(III) an exposure assembly for forming the electrostatic latent image on the image bearing member having primarily been charged;
(IV) a plurality of developing apparatus each for developing the electrostatic latent image by the use of a non-magnetic toner to form a toner image; and
(V) a transfer assembly for sequentially transferring to a transfer medium the toner image formed by each of the developing apparatus;
the developing apparatus comprising;
a toner hopper for holding therein a non-magnetic one-component toner for developing the electrostatic latent image formed on the surface of the image bearing member; and
a developing roller for transporting the non-magnetic one-component toner held in the toner hopper, while causing the toner to adhere to the roller surface;
wherein;
the developing roller comprises;
a conductive mandrel;
a charge-providing layer having a charge-providing performance to the toner, formed at the surface of the roller;
a base layer having an elasticity, formed at a position nearer to the mandrel of the roller than the charge-providing layer; and
an elastic intermediate layer formed at a position between the base layer and the charge-providing layer; and
a composition for forming the elastic intermediate layer has a contact angle to water which is smaller than the contact angle to water of a composition for forming the base layer.